1. Field of the Invention
This invention relates to a grid-type flow distribution baffle for use within a steam generator having an array of heat exchange tubes. The baffle provides lateral support for the tubes, and helps to generate a sludge-sweeping, radially-oriented flow of water over the tubesheet of the generator.
2. Description of the Prior Art
Flow distribution baffle plates are known in the prior art. Generally, such baffle plates are mounted about 20 inches above the tubesheet in the secondary side of a nuclear steam generator, and include an array of bores for receiving the numerous U-shaped heat exchange tubes present in the secondary side of the generator. Such baffle plates are circular in shape, and are mounted around the lower edge of the cylindrical tube wrapper which encloses the U-shaped tubes. A large, centrally disposed opening is disposed in the center of such plates for providing a flow path for the water which flows between the upper surface of the tubesheet and the lower surface of the baffle plate. These baffle plates perform two important functions. First, the tube-receiving bores lend lateral support to the relatively long and flexible U-shaped heat exchange tubes present in the secondary side of the steam generator. Second, these plates divert a substantial portion of the vertically oriented flow of secondary water into a radially oriented flow across the tubesheet of the nuclear steam generator, which in turn entrains (or "sweeps") and removes the particulate matter or sludges which would otherwise accumulate on top of the tubesheet. This second function is important, because even though the U-shaped tubes are formed from corrosion- resistant Inconel.RTM., the corrodants which can become concentrated in such sludges can eventually lead to stress-corrosion cracking of the U-shaped tubes if they are allowed to accumulate on top of the tubesheet.
One of the primary problems associated with prior art baffle plates has been the design of a tube-receiving bore or opening in the plate which provides a sufficient amount of current-diverting flow resistance while preventing crevice boiling from occurring within the plate. Crevice boiling occurs whenever the amount of water flowing in the annular space between the tube and the plate opening falls below a certain minimum. Under such conditions, the relatively hot heat exchange tube boils water away from a particular section of this annular space faster than the surrounding flow of water can reimmerse this region. Such localized crevice boiling concentrates sludges, and hence corrodants, in the annular space between the tube and the opening in the plate, which can accumulate and corrode and ultimately crack the wall of the tube. If the wall of the tube becomes cracked, radioactive water from the primary side of the steam generator will contaminate the non-radioactive water in the secondary side of the generator.
One of the first of these prior art baffle plate designs employed a cylindrically shaped, tube-receiving bore in which the annular clearance between the bores and the tubes was small enough so that most of the vertically oriented water currents flowing upwardly from the tubesheet would not flow through the baffle plate, but would instead be diverted into a radially-oriented flow which travelled between the underside of the baffle and the top surface of the tubesheet. Additionally, the annular clearance between the bore and the tube was large enough so that when the tube was concentrically disposed in its respective bore, a sufficient amount of water flowed in the annular space therebetween to prevent crevice boiling. Despite the intent of this design, a substantial amount of crevice boiling occurred in the annular spaces between the tubes and the bores in the plate due to the fact that the tubes usually extended through their respective bores in an off-center manner. This, in turn, allowed only a very thin film of water to flow between the tube and the cylindrical bore in the region where the tube came into contact with the bore. Because this thin film of flowing water could not absorb the heat transferred by the tube without vaporizing, crevice boiling occurred.
In a later design, the cylindrical bores were replaced with apertures having an octagonal profile. While the octagonal shape of the tube-receiving openings allowed a greater flow of water to occur in the region where the tube was closest to the walls of the openings, some crevice boiling still occurred.
In one of the latest prior art designs (the "mini-broach quatrefoil"), a generally circular bore having four flat lands spaced 90.degree. from one another is used. Such a design goes further in eliminating crevice boiling, since it allows an even greater amount of water to occur between the tube and its aperture in the region where the tube is closest thereto. But some crevice boiling can still occur in the regions where the flat lands come into close contact with the outer walls of the tube. Hence, no plate-type baffle has yet been developed which is satisfactory in all respects.
Grid-type flow distribution baffles formed from an interlocking network of metallic straps or bars are also known in the prior art. Grid-type baffles are relatively simple and inexpensive to manufacture, and are relatively easier to install. Additionally, little if any crevice boiling occurs between the cells formed by the grid bars and the heat exchange tubes captured within these cells, due to the large, water-conducting spaces between the bars forming the tube-capturing cells, and the tubes themselves. But these large current-conducting spaces also prevent such grid-type flow distribution baffles from effectively diverting the vertical flow of water in the secondary side of such steam generators into a sludge-sweeping radial flow. One solution to this problem might be to provide some sort of flange around the edges of the tube-capturing cells which would provide a sufficient amount of current-diverting, fluid resistance to each cell so that the baffle, as a whole, diverted most of the vertical current flow to a sludge-sweeping radial flow. However, such a flange would provide a situs for "point contact" to occur between the tube and the cell, which again would lead to poor wear performance.
Clearly, there is a need for a grid-type flow distribution baffle which is easy to manufacture, but which is capable of diverting the vertically oriented water currents within the secondary side of a steam generator into sludge-sweeping, radially oriented currents. Ideally, such a grid-type baffle should further provide lateral support for the relatively long and flexible U-shaped heat exchange tubes inside such generators without any regions of "point contact" between the outer wall of the tube and the inner walls of the grid cells.